What can I do to stop this?

[DGladius]

I wouldn't find it fair to the group if one person or more comes into a party and doesn't give their full attention to fulfill the duty they signed up for. I wouldn't like it if a dps came into a party eating and only pressing a few buttons, or a tank tanking while eating and only pressing a couple buttons whether or not the content is easy. That's intentionally putting in less effort or not being fully committed. You can finish what you need to do first that can cause distractions then queue up for a group content afterwards.